Heat treat oven woes, brick issue?

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ren01228

Joined
Apr 30, 2018
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Hi guys, I've recently attempted to build a HT oven but am having some issues. Tried reading a bunch of threads on here but have had no luck. I'm going to try and include as much info as possible so sorry if its kind of long and disjointed.


The inside of my oven is 15.5" x 4" x 4.5", so .161 cubic feet. Using an Inkbird PID from amazon. I'm running 120v, 13amps, 1500 watts. The bricks are cemented together with furnace cement. According to my calculations I'm getting something like 9600 w/cu ft which is more powerful than most commercial ovens, the 120v Evenheat KH 414 is ~7200 w/cu ft for reference. Is it possible I made it too powerful which would lead to PID issues??


Its having trouble at higher temps, hasn’t reached anything over 750C and that was after letting it run for about 2 hours. Heats up fairly quickly to about 450c, then really slows down. Took around 18 minutes for it to get from 500 to 600C o_Oo_O. After that it just crawls at an abysmal pace.

I read a bunch of threads including the "Everything you wanted to know about PID's but were too afraid to ask" and thought I had a basic understanding, but when I played with the PID settings (proportional band, integral, derivative values) I still couldn't get it to keep climbing. With a set point of 500C I’m able to stay within +/- 2 degrees, so I’m not sure if the issue is with the PID or not?


So its an insulation problem then? If I cut power at 650C it loses ~1 degree C each second. No idea how this compares to others.


My oven has a sheet metal casing, and to my understanding it's supposed to stay cool to the touch. It's not, it gets really hot after about 45 mins of heating. Stays hot for hours after. I plugged the thermocouple and element exit holes with ceramic fiber to stop any heat from escaping. The door has a piece of ceramic fiber on it as well and seals up pretty well. I don't really feel any heat escaping from those areas when I put my hand nearby. I don’t notice a certain part of the case heating up quicker either, feels pretty uniform.

Even with the holes plugged, would that small of an opening really cause the oven to heat up this slowly?


Makes me worried I may have potentially gotten the wrong bricks? I got them locally from a silverware/metal company (who was trying to get rid of probably a thousand of them) for 1/3 of the price as ordering them online. They were used but were in very good shape, no cracks or chips etc. I was told that they were k-23 soft firebricks. They look correct as far as I can tell and are easy to cut into. Here's a pic: https://i.imgur.com/WL11lLA.jpg


I wired it using this diagram that I found posted on here, with an added fuse between SSR 1 and the toggle switch. I've tripled check the wiring so it's exactly as pictured. Diagram pic: https://i.imgur.com/Ps8Wuyi.jpg


Quick summary: Do these bricks look correct?
Is plugging the TC/element holes with ceramic fiber sufficient? Even if it was poorly insulated I would think with the power ratio it could at least get over 750c??


Basically I’m beyond clueless at this point. I’m leaning toward an issue with the bricks/insulation, which would suck because that means a complete rebuild. Any help or input would be really really appreciated. Thanks in advance :thumbsup:
 
It could be moisture from the cement. How many times have you run it? You could try running it a few times without the enclosure to drive off any moisture.

The bricks look like insulating bricks.

There should be an indicator light on the SSD that shows when the coil is on. As long as that is on, the PID is trying to get it hotter.

Do you have something to measure the wattage? A cheap Kill-A-Watt device can tell you if the coil is producing as much heat as you think it should.
 
Hubert S. said:
It could be moisture from the cement. How many times have you run it? You could try running it a few times without the enclosure to drive off any moisture.

The bricks look like insulating bricks.

There should be an indicator light on the SSD that shows when the coil is on. As long as that is on, the PID is trying to get it hotter.

Do you have something to measure the wattage? A cheap Kill-A-Watt device can tell you if the coil is producing as much heat as you think it should.
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Cemented it last week and ran 4 times I think, each has been at least a couple hours so I wouldn’t think it’s the moisture.

I’m watching it right now and the output light is steadily on but she’s struggling. At 720C right now, it’ll climb a few degrees then drop a couple and repeat that cycle. Any idea why it’d be doing that?

I don’t have one of those, could I somehow figure out with a multimeter?
 
Even with insulation around your bricks, your shell will still get hot. It shouldn’t get as hot as the inside, obviously, but that heat has to go somewhere.

As for taking forever to heat, it could be your power source, or it could be the bricks. Not all “k23” bricks are made the same. You might try coating them with something like ITC-100. That will give you a little more efficiency at the very least. The next best option would be to upgrade to 220v instead of 110v.
 
A multimeter can tell you what voltage and amps you’re pulling. Make sure you’re in fact reading 110V across the leads that energize your coil. If you don’t have a clamp meter, you’ll have to run it in series with the lead and one side of your coil, in order to read current.
 
ren01228 said:
Cemented it last week and ran 4 times I think, each has been at least a couple hours so I wouldn’t think it’s the moisture.

I’m watching it right now and the output light is steadily on but she’s struggling. At 720C right now, it’ll climb a few degrees then drop a couple and repeat that cycle. Any idea why it’d be doing that?

I don’t have one of those, could I somehow figure out with a multimeter?
Click to expand...
The first thing to do would be to measure the resistance of the coil with your multimeter with everything disconnected. Did you make the coil yourself?

Your diagram shows a door contact switch. I have had a similar issue where that switch was turning off the furnace intermittently because the adjustment bolt was not set right. It is probably better to look at the LED of the SSR to check whether it is getting power vs. the output LED on the PID.

Can you post some pictures of the furnace?
 
Those bricks look diffrent that any k23 brick I have seen. Thy should be soft enough that you can carve them with a plastic spoon. And being used and not cracked is a clue. I do t know if any k23 that can be used and not crack when being removed.
 
What is the density of the brick? Can you measure and weigh a spare or an offcut? Good IFB for kilns is likely to be significantly less dense than water: JM23 is 480 kg/m3 and K23 is 513 kg/m3.

Can you bypass the controller and SSR so that the elements get mains power direct? Check the temperature every 5 minutes and see whether it's heating at the same rate as with the controller and SSR. If it heats faster and gets hotter, the problem is the controller/SSR. If it heats the same, the problem is something else.
 
Thanks all :thumbsup: I will definitely try your suggestions this weekend and report back.

Hubert- I ordered the element wire from Krueger and had them coil it for me as well. And that’s a good thought, I will keep an eye on the SSR LED.

JT- I used a round file and flathead screwdriver to carve my channels. It wasn’t the easiest with the screwdriver so I’m not sure how a plastic spoon would fare, I’ll give it a shot tomorrow.

I do have a extra couple pieces I’ll check the density of as well.
 
JTknives said:
Those bricks look diffrent that any k23 brick I have seen. Thy should be soft enough that you can carve them with a plastic spoon. And being used and not cracked is a clue. I do t know if any k23 that can be used and not crack when being removed.
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Do you mean when they were mortared in? I don't think any of my bricks are cracked, and I could easily remove them from the furnace without breaking them as they are not mortared in. If you use mortar, I could see how it would be difficult to get them apart without cracking.

The bricks look like insulating fire bricks to me. I've only seen three different types and they all looked vaguely similar, even though they all had more consistent and finer pores. The OP said they are easy to cut into, so they are definitely not hard fire bricks.
 
Just calculated the density of my bricks, 1026 kg/m3... double checked by putting in water, did indeed sink like a brick.

Resistance of my element is right about where I wanted. I haven’t tested the voltage/amps to the element as I’m pretty much a complete electricity newbie and wasn’t positive how to. Multimeter says 10A max so I didn’t want to do anything stupid without knowing. I tested straight from my electrical outlet and it is putting out 120v, though I’m sure that doesn’t really help much. Gonna check and see if any of my buddies have a clamp meter, if not I’ll just pick one up I guess.
 
Most everything I would have asked has been asked ... at this point I would be most interested to hear what happens if you direct power the elements (bypassing the PID. ). If the light on the controller is solid on, it is I think not likely the controller... but you never know ... there is at least one case recently of an inkbird being faulty...
 
Have you measured the resistance of the coil? That seems to be the easiest thing to do, so I would start with that.
 
ren01228 said:
Yes its at about 9.8 ohms which is around what I was aiming for. Should put me around 1470 watts.
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That's good, you can rule out the coil.

The brick seems heavy. Is there any chance the brick got wet and absorbed water? Maybe it was sitting outside in the rain before you bought it?
 
They were actually outside on a couple pallets when I picked them up. I’ve had them for probably two months at this point.

They’re definitely more dense than the usual bricks people use, the ones that Tim listed the densities of. The bricks that are rated to 3000F are around the same density as mine, so that’s what I’m hoping that’s what they are and not something that can’t handle the heat.

Hoping that wiring the elements directly will show it may be the PID. I’d rather replace that than the bricks. Still have to check with a clamp meter as well too.
 
I’m putting my money on the bricks. Density does not tell everything. Thermal conductivity is what matters which is usually associated density but it’s not always the case. The real k23 bricks should feel like a heavy foam. You can easily snap the brick in half with your hands. If one Accidentally falls on the ground it usually will bust up into 3 or more chunks.
 
ren01228 said:
They were actually outside on a couple pallets when I picked them up. I’ve had them for probably two months at this point.

They’re definitely more dense than the usual bricks people use, the ones that Tim listed the densities of. The bricks that are rated to 3000F are around the same density as mine, so that’s what I’m hoping that’s what they are and not something that can’t handle the heat.

Hoping that wiring the elements directly will show it may be the PID. I’d rather replace that than the bricks. Still have to check with a clamp meter as well too.
Click to expand...
The higher rated brick I have used had much smaller pores, but there might well be other bricks available. Judging from the picture, I think it is more likely that they are 2,300F bricks and got rained on before you picked them up. Can you call the person you bought them from and find out how long they had been sitting on the pallet in the open? I doubt the PID is at fault. Either the brick is the wrong kind, or it is full of water.

Can you run the oven without the enclosure? Or at least take the top off? If you have only run it a few times and it was enclosed, you may not have driven off the moisture. If the bricks indeed contain moisture, it may just condense on the sheet metal and drip right back onto your bricks. How hot did the sheet metal get?

You could also do a separate test and just weigh a brick and bake it in the oven for a few hours and then weigh again. Repeat until the weight does not change anymore.
 
I'm questioning the brick/insulation factor also. My oven is k-23 brick lined with a outer 1/2" cement backer board shell in a angle iron frame. Ive had my oven at 1900* plus and I can place my hand on the outer shell with little discomfort.
 
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